Aggregation and Aggregate Stability

What is Aggregation? Aggregate Stability?

photo of slump test

This picture was taken after a slump demonstration conducted on two soils comparing aggregation between no-till and conventional tillage systems. Photo Credit: Joe Rorick, Purdue University

Soil aggregation is described as how well soil particles are bond together through fungi binding threads based on soil composition and texture. Aggregate Stability is the measured ability of a soil to hold together and maintain structure despite disruptive forces such as raindrops; water and/or wind erosion impacts; shrinking and swelling conditions or tillage.

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Regional Educator ResourcesOther Educator ResourcesTechnical Resources
Title Source Resource type and date Short Summary
Rainfall simulator – soil health demonstration Michigan State University Video Demonstration

Sep 2019

Paul Gross and Dean Baas, from Michigan State University Extension, give a rainfall simulator demonstration. The demonstration covers soil health topics such as runoff, water holding capacity, and aggregate stability.

What are soil aggregates? Soil Science Society of America Extension Webpage

Jul 2019


Aggregate formation is a complex process. Soil aggregates are formed through physical, chemical and biological activity below ground. They are even influenced by human factors, like tilling, walking on the surface, or even how you fertilize your garden.

Soil Aggregate Stability – a soil health physical indicator Ohio State University Extension Webpage

Feb 2018


Soil aggregate stability is an important physical indicator of soil health, which protects organic matter accumulation, improves soil porosity, drainage and water availability for plants, decreases soil compaction, supports biological activity, and nutrient cycling in the soil.

Soil Quality Indicators: Aggregate Stability USDA NRCS Factsheet

Jun 2008

Soil aggregates are groups of soil particles that bind to each other more strongly than to adjacent particles. Aggregate stability refers to the ability of soil aggregates to resist disintegration when disruptive forces associated with tillage and water or wind erosion are applied.
Soil Health Spotlight: Soil structure and aggregation Kansas State University Web Article

Feb 2020

A basic discussion of soil structure and aggregation.
Aggregate Stability – NRCS & others Webpage

Sep 2011


Changes in aggregate stability may serve as early indicators of recovery or degradation of soils. Aggregate stability is an indicator of organic matter content, biological activity, and nutrient cycling in soil.

Soil Quality Indicators: Slaking USDA NRCS Factsheet

Jun 2008


Slaking is the breakdown of large, air-dry soil aggregates (>2-5 mm) into smaller sized microaggregates (<0.25 mm) when they are suddenly immersed in water. Slaking occurs when aggregates are not strong enough to withstand internal stresses caused by rapid water uptake.

Slaking – NRCS & others Extension Webpage

Sep 2011


Slaking indicates the stability of soil aggregates, resistance to erosion and suggests how well soil can maintain its structure to provide water and air for plants and soil biota when it is rapidly wetted.

Rangeland Soil Quality—Aggregate Stability USDA NRCS Factsheet

May 2001

Soil aggregates are soil particles that are bound more closely to each other than to surrounding particles. The stability of these aggregates is critical to soil erosion, water availability, and root growth.
Soil Health 101 Soil Health NEXUS Extension Webpage

Jan 2017


The soil structure is made up of the soil profile, bulk density, and aggregates

Soil aggregation North Dakota State University Extension Webpage



Soil aggregation is part of soil’s structure and function and a good indicator of soil health.

Soil Health Nexus Demonstration How-To: Slump Test Soil Health NEXUS How-to Video

May 2019

Techniques to measure soil structure and aggregation. The more aggregate stability you have, the more stable your soil. Prevents erosion.

Soil Physical Properties Fact Sheet North Dakota State University Extension Factsheet

Not Dated


Soil physical properties affect the behavior of soil and the functional processes required to meet environmental and human needs.

On-Farm Soil Monitoring for Water Resource Protection: Farm Assessment for Water Resource Protection Purdue University Extension Publication
Dec 2003
These five on-farm soil monitoring methods are intended to replace analytical soil tests. These monitoring methods and indicators include: Water infiltration rate, earthworm count, soil aggregate stability, penetration compaction, and plant and crop residue cover.
Youth Soil Quality Lessons and Videos – Components and Physical Properties of Soil University of Nebraska Extension Webpage
Not Dated
The quality of soil depends upon a variety of factors. NRCS has developed Soil Quality Kit Guides, fact sheets and materials for teachers to integrate into their soils curriculum. In addition, a curriculum has been developed to use in the classroom. These materials were created with funds provided by the Nebraska Environmental Trust. YouTube videos supplement these curricula as well.
Soil Stability Purdue Soil Health Education Webpage Video
Sep 2017

Short basic educational video on soil stability using the slake test by Dr. John Graveel, Purdue University.

How to Understand and Interpret Soil Health Tests Purdue University Extension Publication
Jun 2018
Purdue and CCSI did some research using some commercial soil health tests. This document describes some descriptions and guidelines that came out of doing that work.
Soil Water and Aggregation SARE Governmental / Extension Webpage

Not Dated


Processes like erosion, soil settling, and compaction are affected by soil moisture conditions, and in turn affect soil hardness and the stability of aggregates.

Soil Quality Demonstrations and Procedures USDA-ARS Lesson Plans

Jan 2011


Demonstrations and procedures including all of the following and more:
Soil as a Precious Resource, Rainfall Simulator – Soil Erosion, Soil Macroaggregate Scale Model,
Dry sieving to collect soil aggregates, Wet Sieving to measure water stable aggregation (WSA), Soil Aggregates – Edible Stability

The Biology of Soil Compaction Ohio State University Extension Factsheet

Dec 2009


Soil compaction is a common and constant problem on most farms that till the soil. Soil compaction is reduced by the formation of macroaggregates in the soil. Microaggregate soil particles (clay, silt, particulate organic matter) are held together by humus or old organic matter residues and are resistant to decomposition.

Title Source Resource type and date Short Summary
Improving Aggregate Stability Cornell University Extension Factsheet



Soil texture, climate, and the health, quantity, and diversity of soil organisms affect aggregate stability. Field management, including tillage operations, addition of organic amendments, and planting and harvesting methods, can impact both aggregate size distribution and stability.

Soil Quality Information Penn State Extension Webpage

Aug 2012


Healthy soils yield healthy crops, But what is healthy soil and how do we achieve it?

Assessing soil aggregate stability Murray Catchment Management Authority Video by Australian Government Organization

May 2012


Video on how to measure. The aggregate stability of the soil is tested by observing whether slaking or dispersion occurs after the aggregate is added to water. Slaking vs. dispersion. Slaking is the breakdown of an aggregate of soil into much smaller aggregates on wetting, and is caused by the swelling of clay and the sudden release of trapped gases. Dispersion is the separation of soil particles from aggregates so the structure completely disintegrates into individual particles.

Soil aggregate stability Department of Primary Industries & Regional Development, Australia Governmental Webpage

Jan 2018


Soil aggregate stability refers to the ability for a soil to regulate the movement and storage of air and water throughout the soil profile which is determined by the soil make up of sand, silt and clay particles. The more stable the soils aggregates the more productive the soil.

Title Source Resource type and date Short Summary
Soil Aggregate Stability: A Review Journal of Sustainable Agriculture Peer-reviewed Publication

Nov 1998


This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatments in the laboratory.

Evaluation of methods for determining soil aggregate stability Soil and Tillage Research Peer-reviewed Publication

Apr 2017


Different methods for measuring aggregate stability are poorly correlated. Choice of methodology should mimic breakdown of field aggregates.

This page reviewed by Liz Schultheis, Rick Koelsch, Anna Cates, Joe Rorick, Walt Sell, and Chryseis Modderman.

Last reviewed 10/1/19